A control arrangement is known from the German utility patent G 92 00 708.2, which allows for periodically changing controlling and disturbance variables. Processes comprising these variables include stock removal operations such as planing, turning, milling, grinding, boring etc. The periodicity relates not only to the controlling variables, such as the change of the angular position of lathe work, but also to disturbance variables, for example the cutting forces. Additionally, the utility model relates to the coupling of a periodic linear motion with a periodic rotary motion, the coupling of two rotary motions or the coupling of periodic linear motions. Moreover, the prior control setup relates also to fluctuations of synchronism in machine tool drives due to a pole reversal. A tool or workpiece is considered to be a work object.
Conventional control methods do not allow a sufficient suppressing of external disturbance variables thus inviting the risk of instabilities. This relates notably also to the case of equal periodicity of the controlling variable and the disturbance variable.
Described in the book O. Follinger "Regelungstechnik," Huthig-Verlag, Heidelberg, 6. edition, p. 519 with literature references p. 526, is an extension, proposed by C. Johnson, of a so-called Luenberger observer by a disturbance model. In this context, a disturbance estimate, calculated correctly by phase and amplitude, is with inverted sign superimposed on the controlled system, such as is done similarly in the known case of directly measurable disturbances by feeding the disturbance variables forward into the controller. This method is suited favorably in using a digital process control computer. For each scanning step, however, both the mathematical mapping of the controlled system and of the disturbance must be calculated. As the complexity of the disturbance model increases, the continual calculation leads to appreciable calculating effort, which in the case of fast processes, such as in controlling electrical drives in machine tools, can overwhelm typical standard microprocessors. The scanning intervals can become impermissibly high, which invites the risk of instabilities. With the addition of a digital measuring system, such as positional determination by angle transmitter, control can be rendered impossible when fluctuations assuming higher frequencies due to the low scanning frequency are being folded back, since the scanning theorem is violated. In addition to eliminating disturbances, the adjustment of a periodic controlling variable which is free of contouring error also poses difficulties.
The aforementioned German utility model G 92 00 708.2 provides a solution to the described control task which does not require a disturbance variable observer. The solution consists in the fact that for adjustment of the periodic controlling variable and/or for tune-out of a periodic disturbance variable, the controller features at least one conjugate complex pole pair, wherein the amount of each of the two poles is equal or approximately equal to the periodic frequency of the disturbance variable. In practice, the configuration of the feedback loop is such that a disturbance compensation system in the form of a disturbance controller is wired parallel to a controller configured in accordance with conventional aspects. This disturbance controller has the aforementioned properties, which are described by the behavior of an integral band-pass controller or a proportional band-pass controller.
The properties of both controller types are represented by their complex transfer functions, that is, the output of the disturbance controller, based on an input that matches the control difference, is represented by the Laplace transform of the respective variables. The complex function variable s allows illustration by way of the relation s=.delta.+.tau..omega. in a frequency representation that is determined by the amplitude response and the phase response of the controller. Besides, an inverted Laplace transformation makes it possible to achieve a direct representation in the time range.
If in the prior control setup the frequency of the periodic motion changes, the frequency of the disturbance controller of the disturbance compensation system must be adapted. Problems arise when such frequency changes occur within the original period.
Therefore, the objective underlying the invention is to propose an improved control method that eliminates the disadvantages described above and, notably, allows reacting directly to frequency changes of a periodic controlling variable or of a periodic disturbance variable which act upon the movement of a work object being moved along a specific local coordinate.
Furthermore, a control setup for realization of the inventional control method is proposed.